Japanese Patent Application Publication No. 49-17666 (1974) discloses a process for preparing a polybutadiene rubber with enhanced mechanical strength. This process comprises two successive polymerization steps in the first polymerization step a first polymerization mixture containing 1,3-butadiene and a polymerization medium is subjected to a polymerization in the presence of a cis-1,4-polymerization catalyst which has been prepared from a cobalt compound and a halogen-containing organic aluminum compound of the formula: EQU AlR.sub.n X.sub.3-n
wherein R represents a member selected from the group consisting of alkyl radicals having 1 to 6 carbon atoms, a phenyl radical and cycloalkyl radicals, X represents a halogen atom and n represents a numeral of from 1.5 to 2, to produce cis-1,4-polybutadiene, and; the second polymerization in which a second polymerization mixture containing the resultant first polymerization mixture and, optionally, both or either one of additional amounts of 1,3-butadiene and said polymerization medium, is subjected to a polymerization in the presence of a 1,2-polymerization catalyst which comprises (1) a cobalt compound, (2) an organic aluminum compound of the formula: EQU AlR.sub.3
wherein R is as defined above, and (3) carbon disulfide, to prepare a polybutadiene rubber.
The above-mentioned process is disadvantageous in that the organic aluminum compound to be used as a component of the cis-1,4-polymerization catalyst is different from the organic aluminum compound to be used as a component of the 1,2-polymerization catalyst. That is, it is necessary to use two different types of aluminum compounds. This necessity causes the catalyst-preparation procedure to be complicated and costly. Therefore, it is preferable if a single type of aluminum compound can be used as a component of both the cis-1,4-polymerization catalyst and the 1,2-polymerization catalyst.